Signal equalizing method employing test sequences

ABSTRACT

In order to equalize digitally coded signals, predetermined test sequences are transmitted before useful data is transmitted. The test sequences are correlated with an identical test sequence stored in the receiver and the result of the correlation, in the form of a channel pulse response correlation spectrum, is used to control filter coefficients of a receiver filter arrangement to simulate an inverse transfer function of the transmission channel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a signal equalizing method in a receiver fordigitally coded signals received over a transmission channel withmulti-path reception.

2. Background Information

In wireless signal transmission in the higher frequency domain, thesignal sometimes reaches the receiver in different ways, i.e., fromdifferent directions. In addition to a direct path, reflections frombuildings or natural elevations may conduct the signal broadcast by thetransmitter to the receiver on other than the direct path. Since theindividual paths differ in length and attenuation, the receiver receivesseveral signals which all contain the original information but maydiffer from one another in amplitude, delay and phase angle. Thesuperposition of these signals results in a compound signal in which theoriginal information is more or less distorted. This may result inconsiderable interference, particularly in digital signals.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a signal equalizationmethod which eliminates the interferences caused by multi-path receptionto the extent that the original signal is readable again.

This is accomplished in a method in which predetermined test sequencesare transmitted, received and correlated to obtain a channel pulseresponse which is thereafter used to continue a filter arrangement forcorrecting for multi-path distortions.

In an embodiment of the method according to the invention, the transfercharacteristics of the transmission channel are determined in order toobtain the composition of individual transmission paths at the locationof the receiver The determination of these characteristics may berepeated at short time intervals so that changes occurring in themeantime, as they are encountered in particular with moving receivers,can be considered and adjusted to.

For this purpose, test sequences of an agreed-upon structure arebroadcast and, due to the multi-path reception, they are of coursesubjected to the same distortions as useful data. However, in contrastto the useful data, the structure of the test sequences is known fromthe start so that conclusions regarding the distortions that took placein the transmission channel can be drawn. In detail, a correlation isperformed between the test sequence which is also stored in the receiverand the received test sequence which is distorted due to superpositionof multi-path signals.

The result, the channel pulse response, constitutes a correlationspectrum in which the individual spectral components represent the pathson which the original signal traveled from the transmitter to thereceiver. This spectrum can be employed to control a filter arrangementwhich substantially simulates the inverse of the transfer function ofthe transmission channel. Once the superposed signal has passed throughthis filter arrangement, the distortions that occurred on thetransmission path are cancelled out. This filter arrangement may beadjusted after each test sequence and then the adjustment remains ineffect for the reception of useful data until the next test sequence,since it can be assumed that the determined transfer characteristics ofthe transmission channel remain constant for a certain period of time.

Modifications and advantageous features of the method according to theinvention are defined in the claims, the further description of theinvention and the drawing figures which depict one embodiment thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing figures show the following:

FIG. 1 is a multi-path transmission model with equalization at thereceiving end;

FIG. 2 is the configuration of a data frame in which useful data andtest sequences are transmitted;

FIG. 3 is a section of a data stream to illustrate the cycliccorrelation;

FIG. 4 is a cyclic correlation spectrum;

FIG. 5A-5B sow the sum sequences and a sum sequence function derivedtherefrom;

FIG. 6 is a possible filter arrangement for the inverse simulation ofthe transfer function of the transmission channel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, the transmitted signal U_(S) travels over a transmissionchannel which itself includes three transmission paths. Thesetransmission paths differ from one another in their amplitude "a", theirdelay "τ" and their phase angle "φ". The indices represent the ordinalsof the transmission channels. At the receiving end, these threetransmission paths are linearly superposed on one another. In thereceiver, the distortions are removed in that two feedback connectionare provided which carry the inverse transfer functions of two of thethree transmission paths. By feeding the received multi-path signal withthe inverse sign into these inverse transmission paths so that it isadded to itself, a signal U_(S) results which now reaches the receiverover the path marked with the index 1.

In order to determine the transfer characteristics of the channel, testsequences are transmitted before the actual transmission of useful data.A frame structure for this data format is shown in FIG. 2. The lowerportion of the drawing shows a main frame which, for example, includes16 data channels Ch1 to CH16. The first data channel CH1 is shown againseparately in the upper portion of the drawing expanded in time. As canbe seen in the enlarged view, a test sequence is initially transmittedwhich is then followed by the useful data. Preferably, two successive"M-sequences" are suitable as a test sequence since they have excellentautocorrelation and cross-correlation characteristics.

M-sequences are binary sequences of a maximum length 2^(n) -1 that canbe reached with a polynomial of the ordinal n. Such polynomials of theexpression h(x) =h_(n) ·x^(n) +. . . +h₁ ·x +h₀ can be realized by meansof feedback connected shift registers. In this case, h_(n) and h₀ mayequal 1 and the other h_(i) may equal 0 or 1. The degree n of thepolynomial also indicates the number of registers in the shift register.The register outputs represented by h_(i) =1 must be linked jointly withoutput h₀ by means of EXOR gates and must be connected with input H_(n).Before running the shift register, the registers must be set; in no casemust all registers be set to zero.

If the expression u_(i) =(u₀, u₁, . . . u_(N-1)) represents anM-sequence, then T^(i) u is the sequence u cyclically delayed by i clockpulses:

    T.sup.i u=(u.sub.i, u.sub.i+1, . . . , u.sub.n-1, u.sub.0, . . . u.sub.i-1)

Generally, M-sequences have the following characteristics:

a. the period of u is N =2^(n) -1;

b. there are n different phases of sequence u:

    u, Tu, T.sup.2 u, . . . T.sup.N-1 u.

c. The EXOR linkage results in the following:

    T.sup.i u⊕T.sup.j u=T.sup.k u for i≠j≠k 0<=i,j,k<N

d. u[q] is formed from M-sequence u in that every q^(th) bit of u isutilized for a new sequence v (v_(i) =u_(q)·i mod N)·u[q] has a periodduration of N/gcd(N,q), that is, u[q] produces an M-sequence only if qis odd and gcd(N,q) =1 (gcd(N,q) represents the greatest commondenominator of N and q);

For example, n =6, N =63, q =3 →gcd(63,3) =3, period=63/3=21 u[3] is noM-sequence; u[5] with N/gcd(N,q) =63 leads to an M-sequence;

e. the reciprocal polynomial of an M-sequence also produces anM-sequence;

    h'(x)=x.sup.n ·h(x.sup.-1)=h.sub.0 ·x.sup.n +. . . h.sub.n-1 ·x+h.sub.n

    u=h(x) becomes u[q]h'(x)

if q=2^(n-1) -1=(N-1)/2;

f. the autocorrelation spectrum of M-sequences has only two values:

    Q.sub.u (1)=N for 1=0 mod N

    Q.sub.u (1)=-1 for 1 =0 mod N

if -1/1 is employed as the pair of binary values.

In order to keep the correlation spectrum free of components that werecreated by additional superposition of useful data, two M-sequences aretransmitted as test sequences. These two M-sequences are identical. Thelength of the M-sequences is selected so that the maximum difference indelay to be expected on the individual transmission paths is no longerthan that of an individual M-sequence. The superposed signal then has asection in the data stream in which only those signal components arepresent which were created from the superposition of M-sequences andthus contain no additional useful data. This point in time occurs whenthe second one of the transmitted M-sequences just arrives over thetransmission path having the shortest delay, that is, arrives first inthe receiver.

FIG. 3 shows this section which is marked M2. The given M-sequence MVIshown therebelow is intended to indicate that this marked section M2 ofthe data stream is cyclically correlated with the predeterminedM-sequence. The cyclic correlation is effected in several steps whichwill be explained below. First, each byte of section M2 is multiplied bythe bit or byte therebelow of M-sequence MV1. The term byte is mentionedin connection with this section because the additions and subtractionsof the signal components resulting from the superposition are displayedwith their accurate values. The products resulting in this first stepare added together and the result of the addition yields the firstindividual component of the correlation spectrum.

Then section M2 is shifted by one byte relative to M-sequence MV1 oralso vice versa. However, this causes the byte shifted beyond thesection to be toppled as shown by the M-sequence MV2 shown therebelowand to be again multiplied by the first byte of the other data word insection M2 so that again all bytes of section M2 and of M-sequence MV2are included in the multiplication. Addition of the products yields thesecond individual component of the correlation spectrum. These steps arerepeated by way of the also shown third step employing the shiftedM-sequence MV3 until all individual components have been determined.

The mathematical functions describing the M-sequence, the multi-pathsignal, the correlation and the pulse responses are summarized once morebelow. A distinction is here made for the pulse responses as to whetherthe value pair of the M-sequence is -1/+1 or 0/+1:

A possible correlation spectrum resulting for a superposition signalincluding an M-sequence is shown in FIG. 4. The value pairs employed inthe M-sequence are here -1 and 1.

In practice, it is a problem to identify the above-mentioned section M2in the data stream for the cyclic correlation. Although it would bepossible to perform a cyclic correlation permanently, the computationsrequired would be unjustifiably expensive. In principle this is possiblebecause a criterion for the moment of correlation with section M2 couldbe a correlation spectrum in which distinct individual components arepresent while in other variations in which correlation calculations arealso made with useful data, no distinct individual components becomeevident.

In order to permit the identification of the section with the leastamount of computations, a correlation method is performed which as awhole involves significantly fewer multiplication steps. Here, thesuperposed data stream formed of M-sequences and useful data iscontinuously correlated with the stored M-sequences, in each case overthe length of one M-sequence, in that part of the data stream of thelength of an M-sequence is multiplied byte by byte by the storedM-sequences and the multiplication results are added. Then a part isselected from the data stream which is shifted over by one byte and themultiplication and addition steps are performed for this part and theparts following it. In contrast to the above-described cycliccorrelation, this process could be called a serial correlation.

One possible correlation spectrum which results if a signal reaches thereceiver over two paths is shown in FIG. 5a. The four distinctindividual components are created in that the correlation furnishes sucha pulse response whenever there is time coincidence of the M-sequencesincluded in the data stream with the M-sequence stored in the receiver.Since the M-sequences are transmitted twice in succession and reach thereceiver on two paths, four individual components appear.

The resulting correlation values are now multiplied by the correlationvalues obtained previously in the same manner and delayed by oneM-sequence. The products having a negative sign are not considered anddrop out. The product spectrum is shown in FIG. 5b where two individualcomponents occur now. The resulting products are then added togetherover the length of an M-sequence.

The result of this addition leads to a step-like continuous sum functionas shown in FIG. 5c. This sum function reaches its maximum when pureM-sequences are correlated with one another. This characteristic can beutilized for the identification of section M2. Viewed from thedescending edge following the maximum value S_(max), the exact beginningof this section M2 lies the length of one M-sequence ahead of this edge.

Once this section M2 has been found in the data stream, the actualcyclic correlation can be performed in the manner described inconnection with FIG. 3 in order to determine the channel pulse response.

In multi-stage transmission methods, e.g. 4-PSK transmission, phaseshifts on the transmission path may bring about the case that theM-sequences transmitted in the one channel no longer appear in thatchannel but in the other channel. This is the case in a 4-PSKtransmission if a phase shift by 90° occurs. With other phase positions,parts of the M-sequences of the superposed data stream appear in the onechannel as well as in the other channel. If one were to transmitseparate M-sequences for each channel, interferences may affect theevaluation It is therefore advisable to transmit the M-sequences at thetransmitting end only over one channel but to evaluate both channels atthe receiving end.

FIG. 6 shows a filter arrangement that can be controlled by means of thecomponents determined in the correlation spectrum in such a manner thatan approximately inverse simulation of the transfer characteristics ofthe transmission channel is realized. This simulation is initiallycomposed of a matched filter 1 of the illustrated structure It isassumed that the pulse response x₁ of the not yet equalized transmissionchannel has the structure indicated at the input of matched filter 1.The filter characteristic is set by means of coefficients C₀, C₁ and C₂which correspond to the individual components found in the correlationspectrum. After summing up the individual paths, a spectrum x₂ resultswhich includes, for example for three different transmission paths, adistinct main component and ahead of it and behind it, smaller ancillarycomponents.

Spectrum x₂ is now conducted through an equalizer 2 whose coefficientsresult from the obtained modified spectrum x₂ at the output of thematched filter The spectrum x₃ present at the output of the equalizer isthen filtered in such a way that the components disposed ahead of themain component appear to be even more attenuated while, however, thecomponents disposed behind it remain substantially unchanged.

Equalizer 2 is now followed by a feedback equalizer 3 whose structure isalso shown. The coefficients of the feedback equalizer are again theresult of the components of spectrum x₃ at the output of equalizer 2. Atthe output of feedback equalizer 3, a spectrum x₄ appears which containspractically only the main component while the partial componentsfollowing it are practically erased and only the partial componentsahead of it are barely recognizable. However, the value of this maincomponent compared to the adjacent components is so high that anunequivocal distinction can now be made and the transmitted useful datacan be evaluated reliably.

We claim:
 1. A filter arrangement equalization method, for use in atransmission system having a receiver for receiving digitally codedsignals transmitted over a transmission channel, the receiver includingthe filter arrangement, for compensating for interference due tomulti-path reception, the method comprising:transmitting to thereceiver, before transmission of useful data, predetermined testsequences; correlating received test sequences in the receiver with atest sequence identical to the predetermined test sequences transmittedthat is stored in the receiver; and using a channel pulse responseobtained as a result of the correlating step in the form of acorrelation spectrum to control filter coefficients of the filterarrangement in the receiver to simulate an inverse transfer function ofthe transmission channel and thereby compensate for multi-pathinterference; wherein the transmitting step includes transmitting twoidentical M-sequences in immediate succession as test sequences; whereinthe correlating step comprises performing a cyclic correlation of astored M-sequence with a section of a received data stream composed ofuseful data and M-sequences and which, after multi-path reception,includes superpositions that are offset in time, the section beingpredetermined by the respective second M-sequence arriving first in thereceiver; and wherein the stored M-sequence is multiplied byte by byteby the section of the data stream, the multiplication results are addedand, after shifting the section by one byte relative to the storedM-sequence, the multiplication and addition steps are repeated untileach byte of the section has been multiplied once by each byte of thestored M-sequence and obtained addition results yields a sum of seriesof the same length as the M-sequence, the sum series being thecorrelation spectrum.
 2. A method according to claim 1, for use inmulti-stage transmission methods such as 4-PSK transmission, wherein thetest sequences are transmitted in only one channel, while only identicalvalues are transmitted in another channel during the same time span,both channels being subjected to correlation.
 3. A method according toclaim 1, wherein, in order to determine the section, the data streamcomposed of M-sequences and useful data is continuously correlated withthe stored M-sequence, in each case over the length of oneM-sequence;wherein part of the data stream of the length of oneM-sequence is multiplied byte by byte by the stored M-sequence,multiplication results are added, a part is shifted by one byte and isselected out of the data stream, and the multiplication and additionsteps are performed for this selected part and subsequent parts; whereincorrelation values are multiplied by correlation values that are delayedby one M-sequence; wherein the resulting products are added togetherover the length of one M-sequence; and wherein, from a thus producedcontinuous sum function, a maximum value is evaluated, and a section ofthe data stream, having a length equal to the length of an M-sequenceand lying ahead of a descending edge following the maximum value of thesum function, is selected as the section for cyclic correlation.
 4. Amethod according to claim 1, wherein the period of duration of the testsequences is greater than a greater difference in delay due to anon-direct path of the transmission channel.
 5. A method according toclaim 2, wherein sum functions are formed by a combined evaluation ofthe correlation values from both channels that coincide in time.